Image forming apparatus

Abstract

An image forming apparatus includes an image bearing member, a first belt, a driving roller, a first driving source, an inner roller, a second belt, a transfer roller for nipping the first belt and the second belt and for transferring a toner image from the first belt onto the recording material, a downstream roller provided to contact an inner peripheral surface of the second belt on a side downstream of the transfer roller with respect to a recording material conveying direction and for forming a surface, on which the recording material is conveyed, between itself and the transfer roller, and a second driving source for rotationally driving the downstream roller. During transfer in which the toner image is transferred onto the recording material, a surface speed of the downstream roller is set to become higher than an inner peripheral surface speed of the second belt.

What is claimed is: 
     
         1 . An image forming apparatus comprising:
 a rotatable image bearing member configured to bear a toner image;   an endless first belt onto which the toner image is transferred from the image bearing member;   a driving roller configured to drive the first belt;   a first driving source configured to drive the driving roller;   an inner roller configured to contact an inner surface of the first belt;   an endless second belt configured to convey a recording material and configured to form a transfer portion where the toner image is transferred from the first belt onto the recording material;   a transfer roller configured to contact an inner surface of the second belt and configured to nip the first belt and the second belt between itself and the inner roller and configured to form the transfer portion;   a downstream roller provided so as to contact an inner peripheral surface of the second belt on a side downstream of the transfer roller with respect to a recording material conveying direction and configured to form a surface, on which the recording material is conveyed, between itself and the transfer roller; and   a second driving source configured to rotationally drive the downstream roller,   wherein during transfer in which the toner image is transferred onto the recording material, a surface speed of the downstream roller is set so as to become higher than an inner peripheral surface speed of the second belt.   
     
     
         2 . An image forming apparatus according to  claim 1 , wherein the transfer roller includes an elastic layer of 25° or more and 50° or less in Asker-C hardness. 
     
     
         3 . An image forming apparatus according to  claim 1 , wherein during the transfer in which the toner images transferred onto the recording material, a number of rotations of the second driving source is set so that a speed difference of a surface speed of the downstream roller to an inner peripheral surface speed of the second belt becomes 5% or more and 50% or less. 
     
     
         4 . An image forming apparatus according to  claim 3 , wherein the number of rotations of the second driving source is set so that the speed difference becomes 10% or more. 
     
     
         5 . An image forming apparatus according to  claim 1 , further comprising a torque limiter provided in a drive transmitting portion for transmitting a driving force from the second driving source to the downstream roller. 
     
     
         6 . An image forming apparatus according to  claim 5 , wherein the torque limiter is constituted so that during the transfer in which the toner image is transferred onto the recording material, a state in which the downstream roller is driven at a torque within a limit torque is maintained so that the surface speed of the downstream roller becomes faster than the inner peripheral surface speed of the second belt. 
     
     
         7 . An image forming apparatus according to  claim 1 , wherein the second belt is supported by two rollers of the transfer roller and the downstream roller. 
     
     
         8 . An image forming apparatus according to  claim 1 , wherein as viewed substantially parallel to a rotational axis direction of the transfer roller, when
 lengths of two line segments, each including end points which are intersection points with outer diameter circles of the transfer roller and the downstream roller, in two tangential lines circumscribed with the outer diameter circles of the transfer roller and the downstream roller so as not to intersect each other are L 1  and L 2 , respectively,   lengths of two arcs along the outer diameter circles of the transfer roller and the downstream roller, respectively, so as to connect between the end points of the two line segments on a transfer roller side and so as to connect between the end points of the two line segments on a downstream roller side are L 3  and LA, respectively, and   an axis-to-axis distance between the transfer roller and the downstream roller in a case where a length of a sum of L 1 , L 2 , L 3 , and LA is equal to an inner peripheral length of the second belt is L,   the transfer roller and the downstream roller are provided in positions so that the axis-to-axis distance becomes L±5%.   
     
     
         9 . An image forming apparatus according to  claim 1 , wherein during the transfer in which the toner image is transferred onto the recording material,
 when an outer surface speed of the first belt is V 1 , the inner peripheral surface speed of the second belt is V 2 , and the surface speed of the downstream roller is V 3 , the following relationship is satisfied:   
       
         
           
             
               
                 V 
                 ⁢ 
                 3 
               
               > 
               
                 V 
                 ⁢ 
                 1 
               
               ≥ 
               
                 V 
                 ⁢ 
                 
                   2 
                   . 
                 
               
             
           
         
       
     
     
         10 . An image forming apparatus according to  claim 8 , wherein the length of the sum of L 1 , L 2 , L 3 , and L 4  is the inner peripheral length or less of the second belt. 
     
     
         11 . An image forming apparatus comprising:
 a rotatable image bearing member configured to bear a toner image;   an endless first belt onto which the toner image is transferred from the image bearing member;   an inner roller configured to contact an inner surface of the first belt;   an endless second belt configured to convey a recording material and configured to form a transfer portion where the toner image is transferred from the first belt onto the recording material;   a transfer roller configured to contact an inner surface of the second belt and configured to nip the first belt and the second belt between itself and the inner roller and configured to form the transfer portion;   a downstream roller provided so as to contact an inner peripheral surface of the second belt on a side downstream of the transfer roller with respect to a recording material conveying direction and configured to form a surface, on which the recording material is conveyed, between itself and the transfer roller;   a first driving source configured to rotationally drive the transfer roller; and   a second driving source configured to rotationally drive the downstream roller,   wherein during transfer in which the toner image is transferred onto the recording material, a surface speed of the downstream roller is set so as to become higher than an inner peripheral surface speed of the second belt.   
     
     
         12 . An image forming apparatus according to  claim 11 , wherein the transfer roller includes an elastic layer of 25° or more and 50° or less in Asker-C hardness. 
     
     
         13 . An image forming apparatus according to  claim 11 , wherein during the transfer in which the toner images transferred onto the recording material, a number of rotations of the second driving source is set so that a speed difference of a surface speed of the downstream roller to an inner peripheral surface speed of the second belt becomes 5% or more and 50% or less. 
     
     
         14 . An image forming apparatus according to  claim 13 , wherein the number of rotations of the second driving source is set so that the speed difference becomes 10% or more. 
     
     
         15 . An image forming apparatus according to  claim 13 , wherein the number of rotations of the second driving source is set so that the speed difference becomes 15% or more. 
     
     
         16 . An image forming apparatus according to  claim 11 , further comprising a torque limiter provided in a drive transmitting portion for transmitting a driving force from the second driving source to the downstream roller. 
     
     
         17 . An image forming apparatus according to  claim 16 , wherein the torque limiter is constituted so that during the transfer in which the toner image is transferred onto the recording material, a state in which the downstream roller is driven at a torque within a limit torque is maintained so that the surface speed of the downstream roller becomes faster than the inner peripheral surface speed of the second belt. 
     
     
         18 . An image forming apparatus according to  claim 11 , wherein the second belt is supported by two rollers of the transfer roller and the downstream roller. 
     
     
         19 . An image forming apparatus according to  claim 11 , wherein as viewed substantially parallel to a rotational axis direction of the transfer roller, when
 lengths of two line segments, each including end points which are intersection points with outer diameter circles of the transfer roller and the downstream roller, in two tangential lines circumscribed with the outer diameter circles of the transfer roller and the downstream roller so as not to intersect each other are L 1  and L 2 , respectively,   lengths of two arcs along the outer diameter circles of the transfer roller and the downstream roller, respectively, so as to connect between the end points of the two line segments on a transfer roller side and so as to connect between the end points of the two line segments on a downstream roller side are L 3  and LA, respectively, and   an axis-to-axis distance between the transfer roller and the downstream roller in a case where a length of a sum of L 1 , L 2 , L 3 , and L 4  is equal to an inner peripheral length of the second belt is L,   the transfer roller and the downstream roller, respectively, are provided in positions so that the axis-to-axis distance becomes L±5%.   
     
     
         20 . An image forming apparatus according to  claim 19 , wherein the length of the sum of L 1 , L 2 , L 3 , and L 4  is the inner peripheral length or less of the second belt. 
     
     
         21 . An image forming apparatus according to  claim 11 , wherein during the transfer in which the toner image is transferred onto the recording material,
 when a surface speed of the transfer roller is V 4 , the inner peripheral surface speed of the second belt is V 2 , and the surface speed of the downstream roller is V 3 , the following relationship is satisfied:   
       
         
           
             
               
                 V 
                 ⁢ 
                 3 
               
               > 
               
                 V 
                 ⁢ 
                 4 
               
               ≥ 
               
                 V 
                 ⁢ 
                 
                   2 
                   .